How do those walk-through scanners in libraries know if you’re taking a book without borrowing it, especially if those books don’t have metal inserts or anything like that?

Question

How does the library borrowing system work? and how come even if there is no obvious metal insert in the book those gates still know when and when not to beep if the book has or hasn’t been borrowed?

In: Technology

Answer 1

Do you know how if you hold a tuning fork near your mouth and sing the right note, the fork will sing back? The books contain very thin “tuning forks” that look like stickers. The scanner you walk through emits a tone that causes any near by “tuning forks” to ring at that same frequency. The scanner then listens for the resonating forks and sounds an alarm if it hears any. You can’t actually hear the frequency though because it is done with radio frequencies.

Edit: It occurred to me that I didn’t actually answer ops question. Basically when you check in or out the book it runs over a large magnet what wacks the tuning fork into and out of tune.